In recent years, an optical disk called “DVD” has been put into practical use as a large-capacity recording medium. The DVD has at most two recorded layers per side, and is capable of recording on both its sides. A playback apparatus for such a multilayer optical recording disk is required to include the function (layer jump function) of controlling the distance of an objective lens in a focusing direction relative to the optical disk so that, in a case where, in a state in which a focus servo action is proceeding on any recorded layer under reproduction, a request has been made for the reproduction of another layer, the focus servo action may be performed on the target layer.
Meanwhile, the playback apparatus performs the automatic adjustments of focus biases in the respective layers before reproducing the data of the optical disk, in order that the pull-in of the focus servo action may be effected every disk at a position where the quantity of jitter is minimized to afford the best reproduction characteristics.
More specifically, as shown in a flow chart of FIG. 8, before the data reproduction, the automatic adjustment of a servo is first performed for the lower layer (the layer which is nearer to the objective lens, and which shall be called “layer 0”) thereby to set the focus bias of the layer 0 (S501). Subsequently, a layer jump is made for the layer 1 (S502), and the automatic adjustment of the servo is performed for the layer 1, thereby to set the focus bias of the layer 1 (S503).
Besides, a focus jump is made for the layer 0 (S504), and the reproduction of the data is started (S505). Thenceforth, using the set values of the focus biases, differences among individual optical disks can be absorbed to perform reliable layer jumps and pull-in of the focus servo action. Incidentally, although these processes are sometimes preceded by the discrimination of disk media, this process shall be omitted.